Although polylactic acid has been used mainly in the medical fields because it is decomposed in living bodies and has excellent mechanical properties, development to various industrial uses or general uses are expected in terms of environmental protection because it can be also decomposed in the natural environment by microorganisms, etc.
As a method for preparing polylactic acid, a method for preparing polylactic acid by heating lactides, a cyclic ester compound, in the presence of a catalyst to conduct ring-opening polymerization of lactides, is known.
In the polylactic acid obtained through the ring-opening polymerization, unreacted monomers remain. However, since the unreacted monomers cause problems of property deterioration by hydrolysis, etc., it is required to decrease the content of residual monomers in polylactic acid.
For this, in Patent Document 1, a method of removing residual monomers in polylactic acid by transferring liquid polylactic acid after polymerization is finished to a biaxial kneader and reducing the pressure is suggested, and in Patent Document 2, a method of removing residual monomers in pellet-shaped polylactic acid using high temperature nitrogen is suggested.
However, the methods suggested in Patent Documents 1 and 2 required a long process time, and caused a problem in that polylactic acid was decomposed, thus significantly decreasing molecular weight in the process of removing residual monomers. Therefore, there is urgent demand for studies on a method capable of removing unreacted monomers without adversely influencing polylactic acid.